If it works as in past, the AT&T model will come with SIM card, which you can activate from anywhere. You should also be able to replace the SIM. And the Verizon model doesn't have a removable SIM; It's already built in.
Again, that's based on past U.S. pattern. Might vary with new iPad, and maybe depend on where you are. Some carriers in some countries sold locked iPads that were subsidized, but the ones sold at standard retail price (unsubsidized) shouldn't be locked.